Solid-state imaging devices (image sensors) come in two types of a front-illuminated type and a back-illuminated type. Moreover, solid-state imaging devices in which the photoelectric conversion portion is formed of a silicon-based photodiode or an organic material layer are known.
In a solid-state imaging device having a silicon-based photodiode, the photodiode is formed of an impurity-implanted crystalline silicon layer, and incident light is subjected to photoelectric conversion whereby signal charges (for example, electrons) are extracted. Color filters for imaging visible colors are arranged on the upper layer of the photodiode. This solid-state imaging device has the following characteristics. That is, although the photoelectrically converted electrons propagate through crystalline silicon, due to the properties of silicon, the electrons have a high mobility, and the silicon is easy to process and has high mechanical strength. In addition, existing CMOS techniques can be used, and fast and stable driving is possible. The photoelectric conversion portion of CCD image sensors and CMOS image sensors commercially available on the market is mainly formed of a silicon-based photodiode.
On the other hand, in a solid-state imaging device in which an organic material layer is used as the photoelectric conversion portion, since a plurality of so-called organic photoelectric conversion films having sensitivities to respective light wavelengths are stacked so as to disperse light in a vertical direction, it is possible to disperse light without using a color filter (for example, see JP-A-2003-332551). This solid-state imaging device has the following characteristics. That is, since the organic photoelectric conversion film has a greater optical absorption coefficient than silicon, it is easy to decrease the thickness of the photoelectric conversion portion. Moreover, due to such a structure that does not use a color filter and an optical low-pass filter, the efficiency of incident light is better than the solid-state imaging device having the silicon-based photodiode. Moreover, since reception of light and color separation can be realized at the same position, the problem of false color resulting from a difference in light reception position can be obviated. Furthermore, since the organic photoelectric conversion films are flexible, it is possible to create various shapes and colors. In recent years, CMOS image sensors in which organic photoelectric conversion films are stacked have been developed (for example, see JP-A-2005-268476), which enable color imaging.
Organic photoelectric conversion films are used not only in the solid-state imaging devices but also in solar cells. A method of effectively extracting electrons generated by photoelectric conversion using an organic film made of two kinds of organic materials, for example, an organic film in which p-type crystalline microparticles are mixed into an n-type amorphous body is proposed in Japanese Patent No. 3423279, for example.